The disclosed subject matter relates generally to ceramic counter-flow heat exchangers, and more specifically, to ceramic counter-flow heat exchangers with increased thermal efficiency and resilience.
A typical air-to-air plate fin heat exchanger consists of a stack of inter-conductive air flow sections or passages. Hot air and cold air are forced through alternate passages (referred to as “hot” and “cold” passages, respectively, from time to time herein) in order to exchange heat. In a gas turbine blade disk cooling system, the hot air comes from the engine compressor before entering the turbine blade disks and then flows through bleed passages. The cold air is outside air and flows through ram passages in the engine fan ducts. These alternately stacked ram and bleed passages are joined together along a thermally conductive medium called the parting sheet or plate, and heat from the bleed passages is transmitted through the parting sheets/plates to the ram air flow. Of course, heat exchangers could receive air from other sources as well and the teachings herein are not limited to gas turbine management.
The hot and cold passages are similar and each includes an array of cooling fins and frames or closure bars which are positioned on the parting sheets/plates to define each passage. Frames or closure bars are placed along the edges of the passages to support the ends of the parting sheets/plates. In addition to supporting the ends of the parting sheets, these bars close off each passage, except where there is an inlet or an outlet. At and around the inlets and outlets the fins provide support for the parting sheets.
To fabricate a typical heat exchanger, the sheets/plates are stacked one on top of another with fins therebetween, to define alternating hot and cold passages. The assembly may then be placed in a vacuum furnace for brazing. During the brazing process the stack is squeezed so as to force the sheets and fins together.
The above description relates to typical heat exchangers that may be made with what are called “low temperature” materials. High temperature heat exchanger may be required where bleed air operating temperatures exceed the capability of conventional metals (e.g., low temperature materials). However, some aircraft or other situations may require compact heat exchangers including aircraft engine pre-coolers, gas power plant recuperators, and solid oxide fuel cell waste heat recovery, etc. that are in the high temperature range (T>800° C.). Widely used materials which can withstand temperature up to 700° C. include metal superalloys, such as stainless steel, Inconel, and Haynes alloys. Ceramic heat exchanger technology provides a solution to the high temperature requirements and may allow for inlet temperatures up to 900° C., but manufacturing limitations remained.